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Minárik, Hricová: Log Splitter Design and Construction
Marián Minárik, Júlia HricováLog Splitter Design and
Construction
Projektiranje i konstrukcija uređaja za
cijepanje drva
Original scientifi c paper • Izvorni znanstveni rad
Received – prispjelo: 4. 9. 2013. Accepted – prihvaćeno: 14. 1. 2015. UDK: 630*823.13
doi:10.5552/drind.2015.1349
ABSTRACT • Heating by solid fuels, and especially fi re wood, belongs to the most asked and requested types of
heating in cottages. One of the main problems of heating is wood splitting as a fi rst operation. A very useful tool for splitting of wood material is a log splitter. This contribution is focused on how to apply a screw type log splitter in wood splitting. A set of experiments have been realized to get an appropriate shape of the splitting cone, which will provide an improved splitting action. The cone has been manufactured on the basis of the experimental results. The experimental apparatus has been developed to determine the validity of the splitting cone and its geometry design.
Keywords: log splitter, splitting cone, material selection, tool geometry, wood splitting
SAŽETAK • Grijanje na kruta goriva, posebno na drva za ogrjev, pripada među najčešće vrste grijanja u vikend
kućama. Jedan od glavnih problema tog načina grijanja jest potreba cijepanja drva prije loženja. Vrlo koristan alat za cijepanje drvnog materijala jest cjepač drva. Ovaj je rad usmjeren na specifi čnu upotrebu vijčanog tipa cjepača drva koji znatno olakšava cijepanje drva. Klin za cijepanje drva proizveden je na bazi eksperimentalnih rezultata istraživanja. Razvijena je eksperimentalna oprema za cijepanje drva kako bi se ispitala valjanost klina za cijepanje i njegova geometrija.
Ključne riječi: cjepač drva, klin za cijepanje, odabir materijala, geometrija alata, cijepanje drva
1 Authors are assistants at Technical University in Zvolen, Faculty of Environmental and Manufacturing Technology, Department of Mechan-ics and Mechanical Engineering, Zvolen, Slovakia.
1 Autori su asistenti Tehničkog sveučilišta u Zvolenu, Fakultet za okolišne i proizvodne tehnologije, Odjel za mehaniku i strojarstvo, Zvolen, Slovačka.
1 INTRODUCTION
1. UVOD
Wood is an important natural resource, one of the few that are renewable. It is prevalent in our everyday lives and economy, in wood-frame houses and furni-ture; fence posts and utility poles or fi re wood. The anatomical structure of wood affects strength proper-ties, appearance, resistance to penetration by water and chemicals, resistance to decay, pulp quality, and the chemical reactivity of wood. Many mechanical proper-ties of wood, such as bending and crushing strength and hardness, depend upon the density of wood; denser woods are generally stronger.
Wood is a complex polymeric structure consist-ing of lignin and carbohydrates, which form the visible lignocellulosic structure of wood. Minor amounts of other organic chemicals and minerals are also present, but not contributing to wood structure. The organic chemicals are diverse and can be removed from the wood with various solvents. The minerals constitute the ash residue remaining after ignition at a high tem-perature (Kretschmann, 2007).
Heating by solid fuels, and especially fi re wood, belongs to the most asked and requested types of heat-ing in cottages. With increasheat-ing costs of electric energy and gas, the traditional type of fuel comes into atten-tion also by people living in family houses. One of the
Minárik, Hricová: Log Splitter Design and Construction
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main problems of heating is wood splitting as a fi rstoperation.
A very useful tool for splitting of wood is a log splitter. Splitting a log is a hard work, whether you do it with an axe or with sledgehammer and wedge. The cone achieves a high degree of effi ciency because of its shape and weight.
This contribution is focused on how to apply a splitting cone in log splitting. The cone has a tapered conical shape, which extends from a pointed forward end to an enlarged diameter base. A log to be split is positioned contiguous to the pointed end of the cone. As the cone rotates, its threads draw the log onto progres-sively larger diameter portions of the cone, causing the log to split apart. This cone provides an improved split-ting action, and is relatively easy and cheap to build. Moreover, it prevents logs from becoming stuck on the base part. The disadvantage could be the safety of the operator, because with loose clothes there is a risk of getting caught in the rotating cone or the cone can be dangerous if it becomes stuck in the log. Although a good log splitter can save the operator hours of work with a maul, it is not possible to make it 100 percent safe. Many occupational diseases and injuries occur in forestry and wood processing industry (Suchomel et al., 2011), and therefore work safety is very important. A safety zone should be established around the splitter to prevent injury from fl ying splinters of wood.
2 THEORETICAL BACKGROUND
2. TEORIJSKE OSNOVE
Fracture mechanics is a series of models used to describe the infl uence of cracks and defects on material behaviour. The work of Griffi th (Griffi th, 1921) proved that cracks and defects dictate the strength of material more than any other single feature. He arrived at a for-mula for the fracture stress:
(1) where E is Young’s modulus, R the specifi c work of fracture and a the size of the crack that extends through the thickness of the body. It can be seen that σcrack depends upon both stress and size of crack, not just stress alone. The cracking stress is high when the crack size is small and vice versa. There is a critical crack length below which the crack will not run for a given applied stress, and a critical applied stress below which a crack of given length will not run. The lowest possible value for R is the thermodynamic surface free energy ϒ that Griffi th believed was correct for his ex-periments on glass, so he wrote ϒ in place of R.
Fracture mechanics deals with the whole fi eld of stress and strain around discontinuities to determine the critical loads for fracture. Formulae for stress inten-sity K has the general form
(2) where s is applied stress and a is the size of the crack (in general, ‘size’ may be length, half-length, depth of the crack depending on circumstances; for a small
crack in a large plate, a is the half-length of the crack that extends through the thickness of the body). W is some characteristic dimension of the cracked body. Y(a/W) is a non-dimensional ‘shape factor’ that de-pends on the geometry of the body, orientation of crack and the way in which loads are applied.
When a cracked body is loaded, a stress intensity appears around the crack tip and if the body is unload-ed before cracking occurs, K disappears in the same way that stress (or indeed strain) increases or decreases in a fl awless body. However, experiments show that cracking takes place at a critical value of the stress in-tensity factor, called KC, and this becomes another me-chanical property to indicate resistance to cracking. Then, for fracture at stress σcrack,
(3) where KC has peculiar units N/m3/2 .
Comparison of Eq. (3) and the Griffi th expres-sion (1) shows that
(4) where KC is a combination of two parameters, i.e. Young’s modulus E and the specifi c work of fracture R. The attainment of a critical stress intensity factor at fracture is the same as satisfying an energy-based crite-rion determined from the integrated stress and strain fi elds around the crack and in the body generally. Y(a/W) generalizes the Griffi th formula to any type of cracked body.
Stress intensity factors are often written with Ro-man number subscripts I, II or III. The subscripts rep-resent the mode of cracking, i.e. the way in which separation occurs at the crack tip. Mode I refers to crack opening by simple tension; mode II to in-plane shear cracking where the crack faces slide along one another; mode III also refers to shear, but to cracking by out-of-plane (twisting) sliding motion across the crack faces (Figure 1). The notation is sometimes used inconsistently, since a subscript I is often employed to indicate tensile cracking in plane strain (thick plates, where the critical stress intensity factor KIC is least ow-ing to high hydrostatic stresses, thus leadow-ing to con-servative design), whereas it is just as possible to have tensile cracking under plane stress (thin sheets) where toughness is greater owing to less constraint). A further confusion is that critical stress intensity factors are sometimes called the ‘fracture toughness’. Fracture toughness means the specifi c work of cracking R. In
Figure 1 Types of cracks Slika 1. Vrste pukotina
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Minárik, Hricová: Log Splitter Design and Construction
any case, much cracking takes place in mixed mode,meaning that the crack opening has both tensile and shear components, as when the path of cracking is curved (Atkins, 2009; Smith et al., 2003).
Some investigations on the fracture behaviour of wood in relation to its microstructure were reported by Stanzl-Tschegg (2006). The fracture properties of dif-ferent hard- and softwood species have been character-ized by the wedge splitting technique (Tschegg, 1986) as a rather new fracture mechanical method and were correlated with the resulting fracture morphology. With this method, not only the fracture toughness KIC, but also the specifi c fracture energy Gf needed to complete-ly separate a specimen, has been determined. Several parameters, which do not only determine the mechani-cal, but also the fracture properties, have been studied (Stanzl-Tschegg, 2006).
3 MATERIALS AND METHODS
3. MATERIJAL I METODE
At fi rst, the material of the cone has to be chosen. In the previous study (Mečiarová and Minárik, 2012), some aspects in choosing the material were consid-ered:
– conditions of dividing;
– properties and structure of divided wood and its amount (periodicity of cone using);
– machine tool construction (its stiffness); – cone construction;
– material price.
When selecting a material, the most important factors to consider are hardness and ductility, especial-ly in processes with a higher load. In addition, wood fi bres can contain a natural acid, which can cause dam-age to machines.
Based on the above fi ndings, four materials - steels - have been chosen for the cone production: common carbon steel/structural steel (E335), alloy special steel (16MnCr5), martensitic stainless steel (X39Cr13) and high-speed tool steel (HS10-4-3-10). Each of these steels has certain limitations, properties, structure, price and use. The surface quality of pro-cessed wood was not important in this case.
The steels X39Cr13 and HS10-4-3-10 had a bet-ter wear resistance than the steels E335 and 16MnCr5, so they are more suitable in this case. According to dif-ferent total costs of selected materials (Table 1), addi-tional experiments for material selection were carried out. In the experiments, the tooth shape and tool mate-rial have been investigated.
Selected materials and teeth specimens are shown in Table 2. The teeth specimens were pressed into the rotated wood (oak, spruce) on a machine tool (Figure 2). Cutting conditions were selected as follows: spindle speed n = 350 rev/min, depth of cut ap = 2 mm; cutting time tc = 1 min. The tooth behaviour during cone rota-tion was simulated by real condirota-tions in the experi-ment. At the tooth, it comes to wear and forces action. The forces components and their interaction were ana-lysed in detail. The teeth on the cone were designed on the basis of this analysis and the experiment (per-formed by Mečiarová and Minárik, 2012) has con-fi rmed that steels X39Cr13 and HS10-4-3-10 are the most suitable for the cone production.
Splitting is often used with wet wood cutting, therefore the suitable material for cone production is steel X39Cr13. It is a martensitic stainless steel, which resists weak acids contained in wood fi bres. Another advantage of this material is the hardness and abrasion resistance resulting in preserving ductility in product core. The disadvantage (shown in Table 1) could be the higher purchase costs, but with respect to the total costs of the machined cone and its tool life, it is not a pri-mary criterion for its selection.
Secondly, the point angle of the cone needs to be determined. In the experiment, three cone specimens made of E335 structural steel have been used. The cone specimens (Figure 3) with point angle of 40° (cone 1), 60° (cone 2), and 80° (cone 3), were sunk into the spruce wood (Picea abies) with 18 percent humidity (Figure 4). Each cone had a 40 mm diameter and was Table 1 Comparison of the total costs of selected steels
Tablica 1. Usporedba ukupnih troškova odabranih čelika
Steel / Čelik E335
(No. 1.0060) 16MnCr5 (No. 1.7131) X39Cr13 (No. 1.4031) HS10-4-3-10 (No. 1.3207) Material costs / Troškovi materijala
(80 × 200) 7.03 € 15.54 € 20.35 € 27.85 € Heat treatment costs / Troškovi toplinske obrade 0 € 20.75 € 20.75 € 20.75 € Production costs / Troškovi proizvodnje 26.55 € 26.55 € 34.50 € 34.50 € Total costs / Ukupni troškovi 33.58 € 62.84 € 67.65 € 83.10 €
Figure 2 Scheme of mechanisms for the determination of
tooth shape (1 – teeth; 2 – wood: oak, spruce; 3 – drive unit; 4 – fi xture, 5 – linear support)
Slika 2. Shema mehanizma za određivanje oblika zuba
(1 – zub; 2 – drvo: hrast, smreka; 3 – pogonska jedinica; 4 – učvršćenje; 5 – linearna potpora)
Minárik, Hricová: Log Splitter Design and Construction
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applied to 9 wood specimens, making a total of 27 measurements.
While the cone specimens were sunk, the splitting force process was monitored. The results were recorded
by software, which was connected with the universal strength testing machine Testometric M500-100CT with maximum operating force of 100 kN (Figure 5). Work-ing speed was set to 10 mm/min and the measurement was carried out until the wood breakage. The specimen dimensions are shown in Figure 4.
The way how the cone penetrates the piece of wood (shown in Figure 6) and specimen stress behav-iour are specifi c for elastic-plastic fracture mechanics.
4 RESULTS AND DISCUSSIONS
4. REZULTATI I RASPRAVA
The graph (Figure 7) shows the results of splitting force monitoring. Nine measurements were made for each cone and afterwards a representative force devel-Table 2 Teeth specimens
Tablica 2. Uzorci zubi
Type of steel / Vrsta čelika
HS10-4-3-10 (No. 1.3207) E335 (No. 1.0060) 16MnCr5 (No. 1.7131) X39Cr13 (No. 1.4031)
Used for beech tree (
Fagus
sylvatica
)
Primijenjeni za drvo hrasta
Used for spruce (
Picea abies
)
Primijenjeni za drvo smr
eke
Figure 3 Cone specimens Slika 3. Uzorci klina
Figure 4 Geometrical parameters of a specimen Slika 4. Geometrijski parametri uzorka
Figure 5 Cone testing Slika 5. Testiranje klina
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Minárik, Hricová: Log Splitter Design and Construction
opment was statistically chosen by median and plotted into the graph. The graph indicates that the optimal point angle is between 40 and 60 degrees. This fact has been confi rmed by the calculated values of mechanical work, which represents the area below the splitting force curve. The calculation was performed in Micro-soft Excel. The value for cone 1 (point angle of 40°) is represented by mechanical work of 151.3 J. Value 124.6 J belongs to cone 2 (point angle of 60°). The third value 680.8 J was calculated for cone 3 (point angle 80°). This shows that the cone with point angle of 60° (with the lowest splitting force) is the most suitable.When splitting, it is necessary to exert a mechan-ical work that causes the plastic deformation. The cre-ated crack consequently helps to split the log, so it is necessary to design a broken pointed angle. At fi rst, a smaller angle mitigates the penetration in a log and af-terwards a bigger angle fi nishes the splitting. The cone shape was developed based on theoretical fi ndings and obtained experimental values (Figure 8).
Finally, the cone was manufactured. Martensitic stainless steel (X39Cr13) was used as a workpiece ma-terial with dimensions 80 200 mm. At fi rst, a clamp-ing part with a 45 mm diameter and hole with a 24 mm diameter were machined at a lathe. Then, a roughing
0 5000 10000 15000 20000 25000 0 10 20 30 40 50 60 Sp li tt in g f orce / s il a c ij epanj a, N
Cone trajectory / putanja klina, mm
Cone 3
Cone 1
Cone 2
Figure 6 Resolution of forces caused by cone penetration
(F – splitting force, FH – horizontal force, FV – vertical
force, FN – normal force, FT – tangential force, R – resultant force, a - point angle)
Slika 6. Raspodjela sila pri prodoru klina (F – sila cijepanja, FH – horizontalna sila, FV – vertikalna sila, FN – okomita sila,
FT – tangencijalna sila, R – rezultantna sila, a – kut klina)
Figure 7 Splitting force process (cone 1 - point angle of 40°, cone 2 - point angle of 60°, cone 3 - point angle of 80°) Slika 7. Promjena sile cijepanja (klin 1 – kut klina 40°, klin 2 – kut klina 60°, klin 3 – kut klina 80°)
Figure 8 Design of the cone Slika 8. Dizajn klina
Minárik, Hricová: Log Splitter Design and Construction
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process was performed at a lathe with working allow-ance of 5 mm on the conical part. The three holes were drilled and threaded at a drilling machine for engine shaft connection. The screw teeth were produced by milling at a CNC machine tool. They were afterwards grinded (Figure 9).
The log splitter (experimental apparatus) has been developed to determine the validity of the cone and its teeth shape design (Fig. 10). The electromotor has a power of 1.5 kW and the maximum rotational speed is 1420 rev/min. The movement of the splitting cone (connected with the electromotor) into the split-ting process is provided by the frame.
The initial experiments indicate that the material selection and geometry design were determined cor-rectly. Four initial power measurements have been made with beech (Fagus sylvatica) specimens by using MI 2492 POWER Q instrument and the values ranged between 110 and 160 W. The maximum power of the
electromotor is 1.5 kW, so obviously the splitting pow-er is only 11 %.
5 CONCLUSION
5. ZAKLJUČAK
This contribution deals with the design and con-struction of a log splitter. A set of experiments have been carried out to get an appropriate shape of the split-ting cone and its teeth. The cone with point angle of 60° (with the lowest splitting force) was chosen as the most suitable. On the basis of experimental results, the designed cone has been manufactured. The initial ex-periments indicate that the material selection and ge-ometry design were determined correctly.
For the total verifi cation of the log splitter, it is necessary to gather a suffi cient amount of measure-ment data, and this will be the aim of the next study.
6 REFERENCES
6. LITERATURA
1. Atkins, T., 2009: The Science and Engineering of Cut-ting: The Mechanics and Processes of Separating, Scratching and Puncturing Biomaterials, Metals and Non-metals. Elsevier, Butterworth Heinemann, 432p. 2. Griffi th, A. A., 1921: The phenomena of rupture and fl ow
in solids. Philosophical Transactions of the Royal Socie-ty A, 221:163-198.
http://dx.doi.org/10.1098/rsta.1921.0006
3. Kretschmann, D. E., 2007: Wood. Kirk-Other encyclope-dia of chemical technology. New York: John Wiley & Sons, p. 1-59.
4. Mečiarová, J.; Minárik, M., 2012: Konstrukční návrh a výroba kuželového klínu pro štípaní dřeva, Strojírenská technologie, XVII (1, 2): 68-75.
5. Smith, I.; Landis, E.; Gong, M., 2003: Fracture and fa-tigue in wood. West Sussex: John Wiley & Sons. 6. Stanzl-Tschegg, S. E., 2006: Microstructure and fracture
mechanical response of wood. Int J Fract, 139:495-508. http://dx.doi.org/10.1007/s10704-006-0052-0
7. Suchomel, J.; Belanová, K.; Štollmann, V., 2011: Analy-sis of Occupational Diseases Occurring in Forestry and Wood Processing Industry in Slovakia. Drvna industrija, 62 (3):219-228.
8. Tschegg, E. K., 1986: Equipment and appropriate speci-men shape for tests to measure fracture values (in Ger-man), Patent AT-390328.
Corresponding address:
Ing. MARIÁN MINÁRIK, Ph. D.
Department of Mechanics and Mechanical Engineering Faculty of Environmental and Manufacturing Technology Technical University in Zvolen
Študentská 26
960 53, Zvolen, SLOVAKIA e-mail: minarik@tuzvo.sk Figure 9 Manufacturing of cone
Slika 9. Proizvodnja klina
Figure 10 Experimental apparatus (1 – wood specimen, 2
– cone, 3 – electromotor / engine, 4 – frame)
Slika 10. Eksperimentalni uređaj (1 – uzorak drva, 2 – klin,
